The present invention relates to the preparation of organic compounds containing olefinic unsaturation. More particularly, the present process is concerned with a method of converting carbonyl functionality in an aldehyde or ketone to ethenylidene functionality under mild conditions without the formation of substantial amounts of undesired by-products.
It is previously known to convert ketones and aldehydes to ethylene-containing compounds (hereinafter referred to as methylenation process) by the use of a Grignard reagent. According to the two-step process, the aldehyde or ketone is reacted with methyl Grignard to prepare an intermediate alcohol which upon dehydration with acid gives an isomeric mixture of olefins. Additional rearrangement of acid labile compounds can occur during the dehydration step.
The Wittig reaction and modifications thereof avoid the formation of isomeric olefins as in the Grignard reaction, but disadvantageously employs multiple steps, the preparation of a phosphonium salt, deprotonation thereof and subsequent reaction with a carbonyl compound. Aldehydes and enolizable ketones are employed only with great difficulty or may not be employed at all.
It is also known to generate the Wittig reagent in situ. Shen et al., U.S. Pat. No. 4,061,759. Buehler et al., Survery of Organic Synthesis, Vol. 2, 157 (1977).
It is further known to treat carbonyl-containing compounds with a mixture of carbon tetrachloride or carbon tetrabromide and triphenyl phosphine to prepare a .beta.,.beta.-dihaloolefin, R. Rabinowitz et al., J.A.C.S., 84, 1312 (1962).
Recently various organometallic methylenation reagents have also been disclosed. Included are methylene-bridged complexes of titanium and aluminum disclosed by F. N. Tebbe et al., J.A.C.S., 100, 3611 (1978) and titanium metallocycles disclosed by R. H. Grubbs et al., J.A.C.S., 102, 3270 (1980).